The attenuation of an optical fiber includes Rayleigh scattering loss, structural imperfection loss, OH absorption loss, and infrared absorption loss. Of these, the Rayleigh scattering loss occupies about 80% of the attenuation at the wavelength of 1550 nm, including those resulting from density fluctuation and those resulting from concentration fluctuation (See M. E. Lines, J. Appl. Phys. 55, 4052 (1984)).
An optical fiber having a pure silica core which includes substantially no metal dopants such as GeO2 for increasing refractive index is designed to have an optical waveguide structure in which the refractive index of the cladding is made smaller than that of the core by adding fluorine to the cladding. In such optical fiber having a pure silica core, the scattering loss due to concentration fluctuation is reduced since the core contains chlorine (Cl) only and substantially no other dopants than chlorine (See Japanese patent laid-open No. 2005-202440). On the other hand, it is known that the splicing loss can be reduced if the core have a refractive-index profile of ring form (See Japanese patent laid-open No. 2013-61620). In order to attain the ring-form profile, it is necessary to dope the core with dopants for changing refractive index, such as germanium and fluorine.